1. Field of the Invention
This invention relates to a new process for the production of new polyisocyanate preparations substantially containing isocyanate semi-prepolymers based on organic polyisocyanates and certain polyhydroxypolyacrylates, the polyisocyanate preparations obtainable by this process and their use in the production of polyurethanes or polyurethane polyureas by the isocyanate polyaddition process, in particular the production of optionally cellular molded articles based on polyurethanes or polyurethane polyureas.
2. Description of the Prior Art
The production of molded bodies having a compact surface skin and a cellular core by the isocyanate polyaddition process is basically already known (see, e.g., Kunststoffe 60, pages 3 to 7 (1970) or DE-AS 1,196,864). With suitable choice of the starting components, this process can be used to produce both elastic and rigid molded articles. If little or no blowing agent is used in the process, the parts obtained are solid. The starting materials are preferably worked up by the so-called reaction injection molding process (RSG or RIM process). This is a technique of filling in which the liquid starting components are introduced into the mold within, a very short time through so-called force controlled mixing heads. When producing parts for the automotive industry, for example, di- and/or trifunctional polyether polyols, in the molecular weight range of about 3000 to 7000, and low molecular weight diols such as ethylene glycol and/or butane-1,4-diol or aromatic diamines containing sterically hindered amino groups are reacted with polyisocyanates, preferably liquid polyisocyanates or polyisocyanate mixtures of the diphenylmethane series, optionally in the presence of blowing agents (see, e.g., U.S.-PS 4,218,543 or U.S.-PS 4,065,410). The molded parts obtained have excellent mechanical properties but it has been found that their surface occasionally is not completely free from pits and pores. Such surface defects can be revealed by rubbing a colored powder into the molded part, where they show up in a lighter color against a deep black (if graphite is used), intact surface due to a light scattering effect. These surface defects are often starting points for mechanical damage.
These surface defects in the form of pores or pits, also known as pin holes, must often be repaired by filling them with a repair or filling compound and then rubbing them down, in some cases before or after they have been degreased, and then covering the surface with a lacquer coat. The removal of such defects thus involves a considerable amount of labor.
It is an object of the present invention to provide novel polyisocyanate preparations which when used as all or part of the polyisocyanate component for the production of molded articles based on polyurethanes or polyurethane polyureas enables products to be produced without such surface defects, and the molded products obtained have excellent mechanical properties.